The field of thermo-chemical conversion of biomass has been under investigation for centuries but has received considerable scientific attention since the 1980's as a potential source for renewable hydrocarbon fuels.
Preferentially useful fuel products are hydrocarbons with high energy content such as methane, alkanes, olefins and light aromatic hydrocarbons. Synthesis gas (CO+H2) is a less preferred fuel product but is still valued as an intermediate for further chemical processing to liquid fuels and chemicals.
Gasification Versus Pyrolysis:
The gasification of coal and biomass has been undertaken for over a century. Moses disclosed an improved design for a coal gasifier in U.S. Pat. No. 1,727,892 in 1929 with numerous other examples disclosed since then.
It is known that processes for the gasification of coal and lignocellulosic biomass become fundamentally different due to the chemical makeup and thermal behavior of these materials despite similarities in the sequence of thermo-chemical processes within the gasification process.
A review of biomass gasification technology was compiled by Knoef [Knoef H., ed., “Handbook Biomass Gasification”, BTG biomass technology group B. V., NL, September 2005] and is incorporated by reference. In general, the object of biomass gasification is the production of ‘synthesis gas’ (CO+H2) or ‘producer gas’ (synthesis gas plus a small fraction of methane) by the thermally induced destruction of biomass polymers with added steam and/or oxygen to primarily form simple gaseous compounds typified by synthesis gas or producer gas.
It is generally known that all forms of lignocellulosic biomass thermochemical gasification progress through a sequence of: drying (removal of free and bound water); thermal pyrolysis as the temperature of the biomass particle rises from near ambient to above the pyrolysis temperature; and reaction of pyrolysis products up to maximum gasifier temperature in order to produce a low molecular weight gaseous stream. Raising the temperature of reactants to achieve pyrolysis is endothermic and requires the supply of heat. In addition, the formation reaction for synthesis gas (H2+CO) from pyrolysis products is endothermic. The heat required for both reactions is typically supplied by the addition of superheated steam or partial oxidation using oxygen or air. The extension of reaction conditions beyond temperatures required for thermal pyrolysis of the biomass feedstock is to further react pyrolysis products towards synthesis gas or producer gas. This is a key differentiator between pyrolysis processes and gasification processes.
Biomass pyrolysis produces non-condensable gas, water, condensable tars and char. Biopolymers such as cellulose, hemi-cellulose, lignin and others are converted by pyrolysis into gaseous compounds at elevated temperature.
Biomass Tars:
One of the key barriers to commercialization is the tendency of biomass thermo-chemical process systems to form carbonaceous ‘tar’ deposits inside process equipment resulting in lengthy process disruptions and costly maintenance and repair requirements.
An extensive review of biomass tars was undertaken by Evans et al in 1998 [Milne, T. A., Evans, R. J., and Abatzoglou, N., US National Renewable Energy Laboratory report NREL/TP-570-25357, “Biomass Gasifier ‘Tars’: Their Nature, Formation, and Conversion Efforts”, November, 1998] and is incorporated here by reference. A recent review of biomass tars was undertaken by Li and Suzuki [Li, C. and Suzuki, K, “Tar property, analysis, reforming mechanism and model for biomass gasification—An overview”, Renewable and Sustainable Energy Reviews, 13 (2009) 594-604] and is incorporated here by reference.
‘Tars’ are not well defined in the literature despite several efforts to categorize them. The term ‘tar’ is insufficient and misleading with respect to forming deposits as not all tars compounds will tend to form deposits. Generally, biomass derived ‘tars’ are considered to be non-water chemical compounds that condense upon cooling to ambient temperature, including benzene. This grouping of tar compounds is often referred to as ‘gravimetric tar’ and relates to its measurement method.
Efforts have resulted in improved biomass tar classification systems based on thermo-chemical properties, chemical makeup and the ability of chemical species to be identified using gas chromatographic techniques. These efforts have resulted in the sub-classification of tars into five categories, however, the categories are only indirectly related to their general propensity of the tars to form deposits.
Elliott reviewed the composition of biomass pyrolysis products and gasifier tars from various processes and observed the chemical transition as a function of process temperature and residence time [Elliott, D. C., “Relation of Reaction Time and Temperature to Chemical Composition of Pyrolysis Oils,” in Soltes, E. J. and Milne, T. A., ed., ACS Symposium Series 376, Pyrolysis Oils from Biomass”, Denver, Colo., April 1987]. Elliot observed the transformation in the following sequence for tars in biomass gasifiers:

Baker et al observed that increases in process severity results in reduced levels of tars but those tars remaining are increasingly intractable and difficult to decompose [Baker, E. G.; Brown, M. D.; Elliott, D. C.; Mudge, L. K. “Characterization and treatment of tars and biomass gasifiers”, PNL-SA-16148; CONF-880850-19, August 1988].
Tars produced by biomass pyrolysis are considered to be ‘primary’ tars. “Pyrolysis vapors” are produced by pyrolysis only generally contain in excess of 50% tars if the conventional definition of gravimetric tars is used. It is generally understood in the art that the ‘primary tar’ chemical species resulting from biomass pyrolysis are derived from fragments of the biopolymers within the biomass as observed by Evans et al [Evans, R. J. and Milne, T. A., “Molecular Characterization of the Pyrolysis of Biomass. 1. Fundamentals,” Energy & Fuels 1(2), pp. 123-138] which is incorporated by reference.
Bio-oil is produced by the rapid cooling of pyrolysis vapors.
In biomass gasification processes, the pyrolysis vapors are quickly reacted to form synthesis gas or producer gas within the gasifier. Gases exiting the gasifier contain much lower levels of ‘tars’ than biomass pyrolysis vapors as most of the primary tars have reacted to form synthesis gas.
It is generally known that a significant fraction of pyrolysis vapors are light oxygenated compounds such as alcohols, ethers, ketones, aldehydes, carboxylic acids containing zero, one, two or three carbon atoms within their molecular structure in addition to the oxygen containing functional group. Another fraction of pyrolysis vapors are light oxygenated compounds such as alcohols, ethers, ketones, aldehydes, carboxylic acids containing one aromatic group within their molecular structures in addition to the oxygen containing functional group. These ‘light oxygenates’ and ‘light oxygenated aromatics’ are considered to be ‘tars’ within the definition of gravimetric tars. Most of the light oxygenate and light oxygenated aromatic compounds in pyrolysis gas condense to liquids at ambient temperature. Some of these compounds can condense on cooler downstream surfaces to form deposits. In addition, some of these compounds can react with other compounds present to form compounds that can condense on cooler downstream surfaces to form deposits.
Another significant fraction of pyrolysis vapors are oligomers (monomers, dimers, trimers & tetramers) of biopolymers, notably lignin. Non-monomer biomass polymeric fragments are depositable tars because of the high boiling points of these compounds. Trimers and tetramers would generally tend to have higher boiling points than monomers and dimers. It should be noted that these primary tar polymeric fragments are, at least initially, not polyaromatic hydrocarbons (PAH) typical of secondary and tertiary tars. Polyaromatic hydrocarbons containing fused aromatic rings are not naturally present in biopolymers and are formed during high temperature processing. High molecular weight heterocyclic and PAH compounds can be considered to be depositable tars because of high dew points.
It is generally understood that, at and above the pyrolysis temperature, the biopolymer derived oligomers can exist as vapor or aerosols within the pyrolysis gas stream [Lédé, J., Diebold, J. P., Peacocke, G. V. C, Piskoriz, J., “The Nature And Properties Of Intermediate And Unvaporised Biomass Pyrolysis Materials”, p 51-65 in Bridgwater, A., Czernik, S., Diebold, J., Meier, D., Oasmaa, A., Peacocke, C., Piskorz, J., and Radlein, D. editors, Fast Pyrolysis of Biomass: A Handbook, CPL Press, Newbury, UK, 1999] which is incorporated by reference.
Boroson et al observed that char and some minor inorganic components within lignocellulosic feed stocks were catalytic to the formation of char [Boroson, M, Howard, J., Longwell, J., and Peters, W, “Heterogeneous Cracking of Wood Pyrolysis Tars over Fresh Wood Char Surfaces”, Energy & Fuels, 1989, 3, pp 735-740]. Morf et al also observed that biomass char was catalytic to the formation of secondary & tertiary tars from primary tars [Morf P., Haslerb, P. and Nussbaumerb, T., “Mechanisms and kinetics of homogeneous secondary reactions of tar from continuous pyrolysis of wood chips”, Fuel, 81, (7), May 2002, pp 843-853]. Both are included by reference. Morf et al observed higher levels of secondary and tertiary tars as indicated by the increased concentration of naphthalene (a PAH) with or without char present at temperatures exceeding 650° C.
The observations of Boroson et al and Morf et al infer that deposits formed may be catalytic to the formation of more ‘deposit’ especially if char particles entrained in the pyrolysis vapors are captured on the surface of a deposit.
Methods of Tar Reduction
General approaches used to reduce tars (and thus depositable tars) in biomass gasifier output streams including:                cool the gasifier output gas and use conventional gas scrubbing means to effect tar removal;        increase the operating/outlet temperature of the gasifier to enhance thermal destruction in the presence of oxidative gases (such as steam, oxygen or carbon dioxide);        increase the temperature of the outlet gas from the gasifier to enhance thermal destruction in the presence of oxidative gases (such as steam, oxygen or carbon dioxide);        employ heterogeneous catalysts for residual tar destruction in the presence of oxidative gases (such as steam, oxygen or carbon dioxide);        condense tars without forming deposits within the system.        
One method for tar reduction is to remove tars after cooling the gas stream. In practice, fine aerosols have proven to be difficult to remove and often an electrostatic precipitator is required in addition to conventional aqueous scrubber systems operating near ambient temperature. This approach does not mitigate problems of deposit formation on cooler system surfaces or gas cooling heat exchanger surfaces. In addition, aqueous scrubbers create waste streams for treatment and disposal.
Examples of the thermal tar destruction approach by raising the output gas temperature and increase residence time were disclosed by Graham and Barynin, in U.S. Patent Application 20040107638 and also Cordell and Gailer in U.S. Pat. No. 6,120,567 in which a controlled quantity of air is introduced into a secondary chamber to reduce tar concentrations, particularly depositable tar aerosols. Gas temperatures well in excess of 750° C., and often in excess of 1000° C., are required for tar destruction. The teachings of Elliot, Boroson and Morf would tend to infer that temperatures exceeding 750° C. would result in the conversion of any remaining tars to undesirable bi- and poly-cyclic PAH compounds.
Oxygen blown, entrained flow gasifiers can operate with maximum temperatures exceeding 1200° C. which is sufficient to destroy virtually all tars prior to exiting the gasifier. A significant disadvantage with thermal tar destruction (by raising the temperature of the gasifier or heating gasifier outlet gas) is a loss of system thermal efficiency.
Norbeck and Hacket disclosed in U.S. Patent Application 20080312348 a tar reduction method in a hydrogen+steam hydrogasification process. Examples disclosed 5 and 25 second conversion times within a steam hydrogasification fluidized bed at 750° C.-850° C. and 180 psi. This process was non-catalytic and examples indicated 750° C. or 850° C. and 25 second residence time was required to achieve tar levels of <1%. The nature of the tars were not reported, however, based on the teachings of Elliot, Boroson and Mort it is surmised that these temperatures would result in the formation of undesirable bi- and poly-cyclic PAH compounds which would tend to be ‘depositable tars’.
Catalysts have been used in a secondary bed in series with the gasifier for the destruction of tars contained in the synthesis gas or producer gas by oxidation or reaction with hydrogen or steam within the synthesis gas. A recent review of published information on catalytic tar reduction was performed by Gerber and is incorporated here by reference [Gerber, M. A., “Review of Novel Catalysts for Biomass Tar Cracking and Methane Reforming”, PNNL-16950, October 20071.
Mudge et al disclosed in U.S. Pat. No. 4,865,625 (Mudge-625) the use of a catalytic secondary reactor in which air, oxygen and/or steam was injected in a secondary bed to effect tar destruction over supported nickel and other catalysts operating from about 550° C. to 750° C. In the Mudge-625 patent, it was disclosed that producer gas (containing<10% CH4) was produced by oxidative steam gasification in a first bed at about 600° C. to about 800° C. and catalytically treated in a second bed at temperatures as low as 500° C. to 600° C. for tar destruction. Injections of air, oxygen or hot steam into the second bed resulted in tar destruction and acceptable levels of coke produced on the catalyst. In this process, the majority of tars were reacted to form synthesis gas or producer gas in the gasifier prior to entry into the secondary reactor. This approach is not suitable for the reaction of tars produced by pyrolysis without gasification. This is indicated by the failure of catalysts placed in the gasifier (which would expose catalyst to pyrolysis vapors prior to being thermally reacted to form primarily synthesis gas or producer gas).
Of special concern noted in this approach was catalyst deactivation due to the formation of coke in the catalyst which was greatly reduced by the placement of the nickel based catalysts in the secondary reactor rather than the first (steam gasifying) reactor. It was noted by Baker et al the importance of maintaining a minimum steam/biomass ratio and maintaining the secondary catalytic bed at elevated temperature to achieve tar reduction and coke re-gasification to combat catalyst deactivation [Baker, E. G., Brown, M. D., Elliott, D. C., Mudge, L. K. “Characterization And Treatment Of Tars From Biomass Gasifiers”. PNL-SA-16148; CONF-880850-19, August 1988] which is incorporated by reference. A minimum operating temperature of 600° C. was noted by Baker et al as well as the preference for fluid bed versus fixed bed. [Baker, E., Mudge, L. and Brown, M. D., “Steam Gasification of Biomass with Nickel Secondary Catalysts” Ind. Eng. Chem. Res., 1987, 26, pp 1335-1339] which is incorporated by reference.
Simell and Kurkela disclose in U.S. Pat. No. 7,455,705 (Simell-705) a method for tar and ammonia destruction of gasifier output gas using a zirconia catalyst in a secondary catalyst bed when combined with oxygen containing gas additions and operating at a temperature of 500° C. to 900° C.
Ekstrom et al disclosed in U.S. Pat. No. 5,213,587 (Ekstrom-587) the use of a secondary fluidized bed containing a catalyst (and absorbent) of magnesium-calcium carbonate and calcined magnesium-calcium carbonate (and mixtures) to affect the destruction of tars, ammonia, etc from a gasifier output stream with an operating temperature of the secondary stage maintained at between about 600° C. and about 1000° C., preferably 700° C.-900° C. Oxygen is added to maintain bed temperature by partial combustion. It is generally known in the art that alkali earth and alkali carbonates (and their corresponding oxides) catalyze biomass pyrolysis and gasification reactions.
The Mudge-625, Simell-705 and Ekstrom-587 processes utilize catalysts to enhance the destruction of residual levels of tars contained in biomass gasifier output streams (synthesis gas or producer gas) and require the addition of oxygen or steam to oxidize residual tars and/or maintain temperatures well in excess of 600° C., preferably in excess of 700° C. The experience noted in Mudge-625 and subsequent publications clearly indicate that the ‘secondary bed with oxygen or steam additions approach’ is not suitable for pyrolysis vapor streams.
Approaches have been disclosed to condense depositable tars on solid media. Brandl et al disclosed in U.S. Pat. No. 4,936,872 the gasifier product gas cooled in a fluidized bed with solid particles removed from the fluidized bed and later returned into the reactor. Rasanen and Pohja disclosed in U.S. Pat. No. 5,562,744 a method and reactor which allow process gas obtained in gasification to be cleaned and cooled using solid media. Incorporation of a solid media gasification catalyst was disclosed and recycle of media to the fluid bed. Finnish Patent 76 834 and Finnish Patent Application 910 731 disclose methods for removing depositable tars from a gasifier output stream by cooling in a fluidized bed reactor to deposit tar and other compounds onto a solid material placed in a secondary reactor before they reach the cooling surfaces of the reactor. This ‘depositable tar condensation on solid media’ approach requires additional complex heat transfer systems for the cooling of solid media and the removal of deposited tars from the media.
Approaches have been disclosed to remove depositable tars by condensation with organic liquids above the condensation temperature of water. Holter et al disclose in U.S. Pat. No. 4,206,186 a two stage process of cooling and tar removal from gasifier output gas streams by cooling to about 600° C. then using a re-circulating oil wash while maintaining the temperature of the gas stream above the dew point of water.
Boerrigter and Bergmann in U.S. Patent Application 20040220285 disclosed the application of oil wash for tar removal from biomass derived synthesis gas at 600° C.-1300° C. This process is also described in several research reports including [Boerrigter, H., van Paasen, S., Bergman, P., Könemann, J., Emmen, R., Wijnands, A “OLGA Tar Removal Technology Proof-of-Concept (PoC) for application in integrated biomass gasification combined heat and power (CHP) systems”, Report No. ECN-C-05-009, January, 2005] which is incorporated by reference.
The ‘depositable tar condensation with organic liquid’ approaches allow for the use of conventional heat recovery systems, however, produces an organic stream contaminated with depositable tars requiring continuous replenishment or recovery and purification of the organic liquid.
It is generally known that the propensity of any vapor phase chemical compound in a gas stream to condense on a cooler surface is directly related to its dew point. The dew point is related to the boiling point of the compound and its concentration in the gas stream. This is somewhat complicated by the tendency of similar compounds to interact and selectively partition between vapor and liquid phases especially for high molecular weight compounds of similar chemical makeup such as oligomers, tetramers and polymers with a range of molecular weights. However complicated, it is a necessary condition for vapor phase components to condense to a liquid or solid phase in order to form a deposit within process hardware. Aerosols are essentially liquid phase droplets suspended in the gas stream.
The situation is further complicated by chemical reactions which result in higher molecular weight compounds being formed (via dehydrogenation, cracking, condensation or re-polymerization reactions). These reactions can be catalyzed by surfaces (or deposits on surfaces). As noted previously, it is generally known that char is catalytic to the formation of more char from biomass pyrolysis vapors.
However, tar vapors that simply condense to form low viscosity liquids do not form deposits. Exceptions can occur if condensed liquid tars interact with other compounds, particulates or aerosols present in the gas stream. The interactions may be physical or chemical, such as by binding of char particles in order to adhere to a surface or by chemical reaction to increase molecular weight.
Biomass Fast (or Flash) Pyrolysis:
The three primary bio-polymer components of biomass are cellulose, hemi-cellulose and lignin. It is generally known that these will thermally decompose in the absence of oxygen to form gaseous or liquid intermediate oxygenated compounds plus carbonaceous char upon heating over temperature ranges depending on the biopolymer type. This thermally induced self-decomposition is typically referred to as pyrolysis as opposed to gasification.
It is generally known that the rate of heating is important to the proportion and composition of the gas phase formed upon decomposition. Generally, flash pyrolysis is considered to require<0.1 second to heat the biomass particle to above the pyrolysis temperature and remove the pyrolysis gas from the reactor. Fast pyrolysis is considered to require about 0.1 to about 5 seconds while slow pyrolysis of biomass is considered to occur over about 30 seconds with rapid pyrolysis in between. Flash, fast, rapid and slow pyrolysis produce pyrolysis (vapor+gas) gas streams which are high in tars but differ in amounts and chemical makeup.
A review of biomass fast pyrolysis was published by Bridgwater et al [Bridgwater, A., Czernik, S., Diebold, J. Meier, D., Oasmaa, A., Peacocke, C., Piskoriz, J., and Radlein, D., Fast pyrolysis of Biomass: A Handbook, CPL Scientific Publishing, UK, 1999] and in a later review by Bridgwater [Bridgwater, A., “Biomass Fast Pyrolysis”, Thermal Science: Vol. 8 (2004), No. 2, pp. 21-49]. Both are incorporated by reference.
Both fast and flash pyrolysis maximize pyrolysis vapor (tar plus light gas compounds) generation and minimize char formation. The carbon content of the pyrolysis gas/vapor is maximized with faster pyrolysis rates and this is generally desirable when producing a fuel from the gas/vapor stream. The speed of pyrolysis strongly affects the solid char formation rate with char formation is generally understood to be inversely related to speed of pyrolysis:
Char formation: Flash<Fast<Rapid<Slow
Biomass Hydro-Gasification Process:
In U.S. Pat. No. 4,822,935, (Scott-935) Scott disclosed a process performing the hydro-gasification of biomass which was accomplished at atmospheric pressure and low temperature conditions far less severe than required for coal and similar feed stocks. The Scott-935 process is a combined fast pyrolysis and hydro-gasification process within the same reactor.
The Scott-935 process consisted of rapid pyrolysis of solid biomass particles fed to a near-atmospheric pressure fluidized bed containing supported catalyst particles in the presence of flowing hydrogen. The catalyst, typically nickel supported on alumina, was found to react biomass with flowing hydrogen gas to form methane and steam with minor proportions of CO, CO2, char and ‘tars’. Importantly, the reaction to form methane was observed to occur at temperatures of 450° C. to 650° C. and preferably 500° C.-550° C. Fluidizing gas (hydrogen) was pre-heated but not above the temperature of the fluidized bed. Hydrogen gas containing entrained wood particles was not pre-heated. A ‘cooling finger’ was incorporated within the fluidized reaction bed to avoid pre-heating of the wood particle and hydrogen feed stream.
The Scott-935 process temperature range is well below typical gasification temperatures required for synthesis gas or producer gas and is within the range of temperatures used for the pyrolysis of biomass for the production of bio-oil. Also of importance, this was performed without the addition of oxygen or air to provide heat by combustion or partial oxidation within the fluidized bed. Reported gas contact times were typically less than 2 seconds and preferably 0.4 to 0.8 seconds. Scott-935 disclosed a gas residence time of less than 5 seconds, preferably less than 2 seconds and most preferably less than 1 second in the fluidized bed catalytic reactor.
Scott-935 disclosed that low but significant levels of ‘tar’ and ‘char’ were produced; however, the levels of ‘tar’ are still above levels required for commercial use. The lowest levels of ‘tar’ disclosed ranged from 0.4 wt % to ˜1 wt % but typically 5 wt % to 15 wt %. The nature of the ‘tar’ produced was not disclosed except that it condensed upon cooling to near ambient temperature.
All reported trials were performed at or near atmospheric pressure with the preferred temperature range being 500° C. to 550° C. Operation below 500° C. trended towards increased tar and char formation and decreased methane formation. Operation above 550° C. trended towards decreased char and methane formation with increased carbon oxide formation. Tar and char formation rates were strong functions of the Wood Feed Rate/Catalyst ratio (F/C ratio) with an optimum F/C ratio being observed.
The formation of coke on the nickel catalyst was noted by Garg et al but did not appear to deactivate the catalyst (in 30 minute runs) [Garg, M, Piskorz, J., Scott, D. S. and Radlein, D., “The Hydrogasification of Wood”, Ind. Eng. Chem. Res. 1988, 27, pp 256-264] which is incorporated by reference. Catalyst coke levels of less than about 3 wt % did not appear to inhibit the methane formation reaction.
The observations of Mudge et al noted that a similar a nickel catalyst was deactivated in a steam fluid bed gasifier within a few hours [Baker, E., Mudge, L. and Brown, M. D., “Steam Gasification of Biomass with Nickel Secondary Catalysts”, Ind. Eng. Chem. Res., 1987, 26, pp 1335-1339]. Mudge-625 disclosed that carbon on the catalyst in a secondary catalyst bed could be controlled by oxidation with oxygen or gasification with steam.
The impact of performing hydropyrolysis of a biomass feed at elevated hydrogen pressure (over the range 300° C. to 700° C. and up to 70 bar) was studied by Guell et al who noted bio-oil yields decreased by relatively small amounts over the pressure range in a well swept reactor and heating rates of about 1000° K/sec [Guell, A., Li, C. Z., Herod, A., Stokes, B., Hancock, P. and Kandiyot, R., “Effect Of H2-Pressure On The Structures Of Bio-oils From The Mild Hydropyrolysis Of Biomass”, Biomass and Bioenergy, 5, 2, pp. 155-171, 1993] which is incorporated by reference. High total volatiles yield was noted (bio-oil plus gases) for wood pyrolysis at 400° C. to 500° C. and up to about 40 bar. A slight decrease in yield was noted at 70 bar. This would indicate that non-catalytic treatment of pyrolysis vapors with pressurized hydrogen gas alone does not produce synthesis nor producer gas.
There is a clear need for a process which converts biomass into a useful fuel which avoids the formation of deposits downstream of thermochemical reactors.